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Setting Up RAID Level 5

Check if the linux kernel already supports software RAID driver
```
 
# more /proc/mdstat
Personalities : [raid5]
read_ahead 1024 sectors
unused devices: <none>
```
However, if RAID level 5 is compiled as a module, the Personalities might be empty. You should try to insert RAID level 5 module
```
 
# insmod raid0
```
and eventually see the same result.
Edit the configuration file /etc/raidtab to add a new RAID device
An example for RAID level 5 is shown below.
```
 
raiddev  /dev/md0
    raid-level              5
    chunk-size              32k
    parity-algorithm        left-symmetric
    persistent-superblock   1
    nr-raid-disks           4

    # Spare disks for hot reconstruction
    nr-spare-disks          0

    device                  /dev/sdb1
    raid-disk               0

    device                  /dev/sdc1
    raid-disk               1

    device                  /dev/sdd1
    raid-disk               2

    device                  /dev/sde1
    raid-disk               3
```
where /dev/sdb1, /dev/sdc1 and /dev/sdd1 are free partitions with Linux native or Linux raid autodetect types. The left-symmetric is the parity-algorithm that offers maximum performance on typical disks. Since when you traverse the striping units sequentially, it allows each disk to be accessed once before accessing any disks twice. Left-symmetric and left-asymmetric algorithm are demonstrated in Figures 13.5 and 13.6. See man raidtab for further detail.

Figure 13.5: Showing how Left-symmetric algorithm operates
$\begin{figure}\begin{center} \epsfig{file=pics/symmetric.eps,height=2in} \end{center} \end{figure}$

Figure 13.6: Showing how Left-asymmetric algorithm operates
$\begin{figure}\begin{center} \epsfig{file=pics/asymmetric.eps,height=2in} \end{center}\end{figure}$
Initial new RAID device
All previous data in disk array will be destroyed then RAID superblock is created.
```
 
# mkraid --really-force /dev/md0
```
The RAID device /dev/md0 must be specified in /etc/raidtab.
Start the new RAID device
The command mkraid also start the RAID device being initialized automatically. Check the started RAID by:
```
 
# more /proc/mdstat
Personalities : [raid5] 
read_ahead 1024 sectors
md0 : active raid5 sde1[3] sdd1[2] sdc1[1] sdb1[0] 16780800 blocks 
level 5, 32k chunk, algorithm 2 [4/4] [UUUU] resync=10.1% finish=588.4min 
unused devices: <none>
```
Size of the RAID device is reported by the number of blocks (each block has 1024 bytes). The 4/4 means 4 out of 4 disks work normally. If there is a corrupted disk, a letter U in UUUU will be replaced by the letter _ indicating the corrupted disk, for example 4/3 UUU_. When the RAID device is successfully created, the syncronization process begins. The /proc/mdstat shows the progres of syncronization and the approximated remaining time. During syncronization, RAID device can be used for reading/writing but redundancy is not guarunteed.
However if this is not the case, you can start RAID device manually by command
```
 
# raidstart /dev/md0
```
Create a file system on RAID device
The RAID device will appear to users just like a typical disk partition. The follow command creates ext2 file system on the RAID device.
```
 
# mkfs /dev/md0
```

**Figure 13.5:** Showing how Left-symmetric algorithm operates
$\begin{figure}\begin{center} \epsfig{file=pics/symmetric.eps,height=2in} \end{center} \end{figure}$

**Figure 13.6:** Showing how Left-asymmetric algorithm operates
$\begin{figure}\begin{center} \epsfig{file=pics/asymmetric.eps,height=2in} \end{center}\end{figure}$

Mount the file system into the directory tree

 
# mount /dev/md0 <mount point>

Finally, you must edit the /etc/fstab to make the modification persistent as shown below.

 
LABEL=/              /                    ext2    defaults        1 1
none                 /proc                proc    defaults        0 0
/dev/sda5            swap                 swap    defaults        0 0 
/dev/md0             /RAID5               ext2    defaults        1 2

Next: Performance Tuning Up: RAID: Redundancy Array of Previous: Setting Up RAID Level Contents

root 2002-09-30